Project Summary Atrial Fibrillation (AF) is the most common cardiac arrhythmia during which the normal, organized rhythm is disrupted and replaced by irregular wave propagation which compromises the primary mechanical function of the heart. It is a serious health concern that, if left untreated, can lead to increased morbidity and even mortality. With the aging of the population, it is a growing problem that affects millions of Americans and represents a considerable economic burden. Unfortunately, the existing therapies have limited success, mostly because the mechanisms that initiate and maintain AF remain poorly understood. Multi-electrode arrays, which can record unipolar electrograms at many discrete sites, are increasingly used to determine the spatio-temporal activation patterns during AF and have revealed the presence of multiple rotational and focal sources of activation. Targeting these sources with localized ablation results in acute termination in roughly 30-35% of patients but can result in long-term freedom of AF that is much higher than other therapies (75-80%). It is currently not clear why some patients do not terminate acutely, which source drives AF, and which source should be targeted first. We propose to develop a quantitative analysis of the temporal stability and the strength of the rotational and focal sources. We will use an existing and extensive data set of unipolar recordings that used a 64-pole basket electrode that covered most of the atrium. We will consider two groups of patients: one in which targeted ablation at sites of rotational or focal activity terminated AF and one in which it did not. We will construct phase maps using a methodology based on a previously published study and will develop algorithms that can quantify the strength and temporal stability of the sources. A comparison between the two groups will allow us to determine whether the elimination of sources with greater strength or stability can predict the result of targeted ablation. Our findings will then be tested in a validation cohort. Our study should result in improved targeted ablation strategies, enabling and advancing the development of better therapeutic strategies for AF.